In a conventional EGR cooler, as shown in FIGS. 7A and 7B, a large number of tubes 4 are disposed in an interior of a shell 7 which is formed into a large-diameter angularly cylindrical shape, and interiors of the tubes 4 constitute gas flow paths, while a space defined between the shell 7 and the tubes 4 constitutes a cooling fluid path. The gas flow paths and the cooling fluid flow path are joined together so as to maintain gas and fluid tightness therebetween.
A cooling fluid inlet pipe 11 is attached to a lower surface portion of the shell 7, while a cooling fluid outlet pipe 12 is attached to an upper surface portion of the shell 7, whereby a cooling fluid passes through the interior of the shell 7 from the cooling fluid inlet pipe 11 to the cooling fluid outlet pipe 12.
Additionally, an inlet header 2 and an outlet header 3 are attached to both longitudinal ends of the shell 7, and exhaust gases flow from the inlet header 2 while being divided into the large number of tubes 4 and are discharged from the outlet header 3.
In a core portion 1 of the shell 7 where the tubes 4 are housed, heat exchange is performed between the exhaust gases and the cooling fluid via the tubes, whereby the exhaust gases are cooled.
As shown in FIGS. 7A, 7B and 9, the tube 4 is a flat tube which is a combination of a tube inner 5 and a tube outer 6 which are disposed to face opposite each other. In order for the tubes 4 to be disposed stacked one on another with a space maintained therebetween, swollen portions 5a, 6a which are swollen in a thickness direction are formed at inlet portions and outlet portions thereof (Patent Document 1).
Additionally, as shown in FIGS. 8A to 8C, an inner fin 8 is housed in an interior of the tube 4 so as to be joined thereto to thereby increase a heat exchanging area so as to promote the heat exchange.